Sony's Alpha SLT-A33, and its stable-mate the SLT-A55, are two of the most interesting and exciting cameras around right now. They may resemble conventional DLSRs from the outside, but rather than having a mirror which flips up when taking a photo, they employ fixed semi-reflective mirrors. Note, Sony describes the mirror as being translucent, but since most definitions associate this with diffused light, like a frosted bathroom window, we believe semi-reflective is technically more accurate.

The semi-reflective mirror in the SLT cameras allows most of the light to pass through to the main sensor, but crucially reflects a smaller portion upwards to a traditional phase-change autofocus system. This gives the SLT-A33 and A55 a unique advantage over traditional DSLRs: rather than relying on slow and laborious contrast-based autofocus during Live View and Movie modes, they can use their much faster phase-change AF systems instead. Better still, the phase-change AF system allows the camera to quickly and continuously autofocus while filming, which is something that's eluded most DSLRs and prevented them from becoming viable replacements for a camcorder.

As we discovered during our extended test period with the SLT-A33, many of Sony's claims are borne-out in practice. It really does offer quick and continuous autofocus for Live View and movies, and in that respect significantly outperforms existing DSLRs. But before you think this may be the answer to all your problems, the concept of combining 100% Live View composition with phase-change AF is not without some downsides.

But we'll start with the positive stuff: in use, the autofocus on the SLT-A33 feels very snappy, and is as quick as focusing with the optical viewfinder on an entry-level to mid-range DSLR. This shouldn't be surprising as both are using similar phase-change AF systems, but the big difference on the A33 is you're composing electronically with Live View. Limitations with Live View on most DSLRs really makes the technology feel like it's getting in the way for normal use, whereas on the A33 it simply lets you get on with taking pictures. It is one of the most positive Live View experiences you'll have.

Of course some may question the benefits of composing still photos in Live View at all - and we'll come to that in just a moment - but you have no choice when it comes to composing and filming video. This is where the AF system in the SLT-A33 really scores, as the full power of the phase-change AF sensor is active throughout. Not only is it quick and continuous, but like other phase-change systems, it knows which direction to turn the focusing. Under ideal conditions this means it can refocus onto subjects without visibly searching back and forth - it just snaps onto them and stops with the minimum of fuss. While some searching can still occur with the A33, it has the potential to be visibly much more discreet than the focus hunting of even the best contrast-based systems.

Compared to video-equipped DSLRs, it feels like a revelation. Models like the Nikon D3100 and D7000 may claim to offer continuous AF while filming, but in fact they just perform a laborious contrast-based search back-and-forth every few seconds - and the result can be very distracting. Other models let you update the focus while filming with a half-press of the shutter release, but again employ the same slow and distracting hunting process. Some don't even bother and demand manual focus while filming - which is what most of us end up doing anyway with the others just to avoid the distraction of searching.

In contrast, the SLT-A33 quickly and confidently autofocuses while filming with continuous adjustments. There are times when tricky subjects or lighting cause it to search, but on the whole it delivers a far superior video AF experience than any traditional DSLR to date. In this respect it's a triumph, and many will buy it on that basis alone: the benefit of a large sensor and broad catalogue of existing lenses coupled with quick, continuous AF and image stabilisation on all of them - see our Sony A33 Movie page for a full report.

But as mentioned above, the semi-reflective system is not without its downsides for both stills and video. Most obviously, the fixed mirror always reflects some of the incoming light away from the main sensor to drive the AF system. We understand about 30% of the light is lost to the AF sensor, which makes the A33 less sensitive than a DSLR employing the same imaging sensor. This means the A33 has to apply greater amplification to the signal to deliver the same ISO values, which in turn means higher noise levels. Some owners have also reported ghosting issues, especially with very high contrast subjects like bright lights on a dark background or stars, although we didn't experience any problems with our standard gallery and test shots.

A lesser-known problem involves the popular perception that phase-change AF systems are infallible. The simple fact is they're not perfect and rely on exact manufacturing tolerances and calibration - otherwise the AF system could be reporting perfect focus when the actual image on the main sensor is fractionally out-of-focus. This is an area where a contrast-based system can take the lead as it typically takes a reading directly from the main imaging sensor, so if it says it's in focus, it generally really is. Indeed when testing the SLT-A33 alongside the contrast-based Lumix GH2, we found the Sony suffering from several AF inconsistencies which just weren't happening with the Panasonic - see our Sony A33 Features page for more details.

Before moving on, it's also worth mentioning a couple of additional foibles concerning phase-change AF on a 100% Live View system. First, the A33's face detection software may track faces all around the frame, but will only actually focus on them if they fall over one of the 15 fixed AF points. Since phase-change systems also only work when the lens aperture is at (or close to) its maximum, the A33 is forced into manual focus if you want a larger depth-of-field for your movies. Finally, many Alpha mount lenses focus quite audibly and unlike most mirrorless rivals, this noise will be picked-up by the internal microphones.

Then there's the pros and cons of Live View composition, especially with an electronic viewfinder. On the upside, the SLT-A33's electronic viewfinder delivers a bigger image than seen through the optical viewfinders of similarly-priced DSLRs, with the additional benefits of magnified focusing assistance and super-imposed graphics like a dual-axis levelling guide.

But like all electronic viewfinders to date, there's never any doubt you're looking at an electronic image. Bright highlight details can often become saturated into pure white areas, low light scenes can become noisy or jerky, and despite the A33 having a high resolution panel, there's still only a finite amount of detail visible. The display technology which switches the colour of each dot to simulate a full colour image can also result in tearing / rainbow artefacts with fast motion: quickly pan the camera or glance from one side of the image to the other and some people will notice coloured edges to objects. To be fair some notice this more than others, but those who do will find it quite distracting at times.

All 100% Live View cameras to date also suffer from a performance issue during fast continuous shooting, where the image displayed between frames isn't live, but the previous shot played-back. This can make it hard to track action as the only feedback you have is the previous shot taken, which doesn't show what the actual subject is doing right now. In contrast the optical viewfinder of a traditional DSLR gives a brief glimpse of what's actually happening between frames, allowing you to easily follow the action and recompose as necessary.

While you can compensate for this to some degree when shooting action with a Live View system, it only works with predictable subjects. If the action is varying in speed or direction it becomes difficult to keep it centred in the frame. If you drop the A33's shooting speed to low, it has sufficient processing muscle to offer a brief glimpse of live action between frames, but at its top speeds, you'll only have the previous frame to guide you.

This is a shame since one of the highlights of the SLT-A33 is its fast continuous shooting speed. It delivers its quoted 6fps in practice, which can be boosted to 7fps if you're happy for the aperture to be locked at its maximum. This is much faster than most cameras at this price point, and while following unpredictable action with Live View is tricky, the speed can be exploited for more predictable subjects, or simply maximising your chance of grabbing the right expression in a portrait - especially useful for kids.

Sony also exploits the fast continuous shooting in its array of innovative multi-frame modes which quickly capture a series of handheld shots before combining them in-camera into a single image. The SLT-A33 offers the same Handheld Twilight, Auto HDR and Sweep features we've seen on earlier Sony cameras which combine multiple frames to reduce noise, increase dynamic range or create broad panoramas respectively. You can see detailed examples of Handheld Twilight and Auto HDR in our results pages.

These modes would be a valuable addition to any camera, but Sony's gone one step further on the A33 with its new Multi Frame Noise Reduction capability. Like the Handheld Twilight mode, it captures and combines six frames into one to reduce noise, but rather than limiting you to fully automatic exposures you can now apply it at any ISO and in any of the PASM modes. It's the icing on the cake and as you'll see in our Sony A33 Multi Frame Noise Reduction results page, it really delivers much cleaner results at high ISOs.

This gives the A33 a key advantage over traditional cameras, even when its semi-reflective mirror is always stealing a portion of the precious incoming light. In our Sony A33 Results pages you'll see the Sony roughly matching Canon's 18 Megapixel models on noise levels (using normal single-frame modes) while also delivering similar levels of real-life detail with their respective kit lenses. Start using the A33's various multi-frame modes though and you'll see it take the lead over comparably-priced (and sometimes more expensive) rivals.

Better still, Sony's responded to previous complaints that its clever modes weren't implemented in the full Auto modes. Now the SLT-A33 offers a more sophisticated Auto+ option which can select Handheld Twilight or Auto HDR in addition to a selection of scene presets. So even beginners or those who simply prefer an easy life can enjoy the benefits of the clever technology.

Now before our final verdict let's see how it compares to some key rivals.

Compared to Sony Alpha SLT-A55

Sony's Alpha SLT-A55 is the closest model to the SLT-A33, and upgrades several respects for a slightly higher asking price. Most obviously the SLT-A55 sports a higher resolution sensor: 16.2 Megapixels compared to 14.2 on the A33.

The main Continuous Shooting speed remains 6fps, but switching to the Continuous Advance Priority AE mode (which uses automatic exposures and locks the aperture to its maximum), sees the speed increase from the 7fps of the A33 to a very impressive 10fps on the A55. This is significantly quicker and gives the A55 a big advantage when it comes to shooting certain types of action. As discussed above, the A55 is equally limited by its lack of live feedback when shooting in its fastest modes which makes following unpredictable action difficult, but if the subject is moving predictably or towards or away from you, it's quite usable. Likewise if the camera's mostly static during the burst, like for many skate, bike and board sports. And again as discussed above, it's also ideal for maximising your chance of capturing the perfect moment in a portrait, especially of kids.

The SLT-A55 isn't just quicker than the A33, but also doubles its buffer, so rather than a paltry 14 best-quality JPEGs in a burst, you'll enjoy closer to 28. The increase in speed and buffer size makes the A55 much more tempting to action shooters than the A33, even with the continued absence of live feedback between frames.

Interestingly the SLT-A55's quoted battery life is a little longer, and we understand its movie mode doesn't crop the image as obviously as the A33.

In terms of new features, the SLT-A55 has one major benefit over the A33: built-in GPS which can record your position and elevation, not to mention the accurate time on images.

So with the SLT-A55, you get 2 extra Megapixels, built-in GPS, a larger buffer for bursts, and faster continuous shooting in the aperture-locked mode. This will cost you roughly 20% more than the A33, which could easily buy you the A33 and a separate handheld GPS for geo-tagging. But if you'd exploit the faster continuous shooting or larger buffer of the SLT-A55, not to mention enjoy the convenience of having the GPS built-in, it's worth spending the extra.

Compared to Panasonic Lumix DMC-GH2

Panasonic's Lumix DMC-GH2 is arguably the biggest rival to the Sony SLT cameras, as each are hybrid models designed to deliver decent stills and HD video without the compromises of traditional DSLRs.

While both the SLTs and GH2 employ 100% Live View from their main sensors though, their approach to autofocusing is entirely different. The Lumix GH2 employs a mirrorless optical path with a fast contrast-based system taking AF readings from the main sensor. The Sony SLTs employ a fixed semi-reflective mirror in the optical path which diverts some of the light to a traditional phase-change AF sensor, which unlike a DSLR, can remain active even while photos or video are being recorded by the main sensor.

Each approach supports quick and continuous autofocus while filming video, although there's pros and cons to both systems. In our tests we found the Sony SLT cameras generally locked-onto subjects with minimal or even no visible searching back and forth, but the motors on the kit lenses were quite audible while doing so. In comparison, the Lumix GH2 visibly searched a little more, but its lenses - including the cheapest 14-42mm kit zoom - did so almost silently. Tipping the decision a little in the GH2's favour was its touch-screen which allowed easy focus-pulling between subjects while filming by simply tapping on them. Countering that though is the fact the SLT-A33 can not only use the entire Alpha lens catalogue, but also support continuous autofocus and stabilisation with each and every one. The bottom line though is both the SLT cameras and the Lumix GH2 delivered far superior continuous autofocusing to a traditional DSLR while filming.

In terms of AF for still photos, the Lumix GH2 was noticeably quicker and quieter than the Sony SLTs, and since it was using data from the main sensor, it also avoided any front and rear focusing errors - which we did notice from time to time on our Sony A33 sample.

But quicker AF for still photos doesn't necessarily make the GH2 the preferable camera for action. With continuous AF enabled, we found it struggled to deliver more than 2fps, while the Sony A33 managed just under 6fps under the same real-life conditions. This makes the Sony SLTs much better for fast action, although both them and the GH2 suffered from a lack of real-time feedback in-between frames when shooting continuously. This really rules them all out for effective action photography where the subject pans quickly or moves unpredictably.

As 100% live view cameras, the quality of the screen and electronic viewfinder are critical, and again there's pros and cons to each camera. Panasonic has opted for 3:2 shaped panels for both the GH2's screen and finder which are a good compromise for images which could be 4:3, 3:2 or 16:9 in shape. In contrast, Sony has opted for 16:9 panels, which are obviously preferable when shooting 16:9 content, but deliver smaller image areas in 3:2. It's also worth noting the GH2 and SLT viewfinders are roughly the same width, so when shooting 16:9 content, the image is displayed at roughly the same size on both cameras; so the Sony only has a size advantage when shooting 16:9 on its main screen.

We also found the GH2's Live View finder suffered from less rainbow tearing than the Sony SLTs, although this can vary from person to person, and in some consolation, the Sony's main screen is higher resolution. Then there's the touch-screen aspect of the GH2, which some may not be bothered by, but others will love; certainly being able to pull-focus while filming by tapping on the desired subject is a neat trick. The final comment on the screens regard their articulated mountings: both can flip and twist to any angle, but we found the hinge at the bottom of the Sony less convenient and intuitive than the one on the side of the GH2; for example, the Sony screen could be blocked when mounted on a tripod.

In terms of size and weight, the Sony SLTs may be more compact than a typical DSLR, but the GH2 comes in a little smaller and lighter with a similar kit lens, without compromising ergonomics.

At the time of writing, we hadn't formally tested the A55's image quality, so can only comment on how the cheaper A33 compared to the GH2. In our tests the A33 delivered very good results, although was unsurprisingly out-resolved by the GH2 as you'd expect when comparing 14 Megapixels against 16. In low light at higher ISOs, you'd expect the bigger APS-C sensor of the A33 to easily outperform the GH2, but the light loss of Sony's semi-reflective mirror coupled with a surprisingly low noise sensor in the Panasonic, make them a reasonably close match. Indeed at higher ISOs, the output from the GH2 was often preferred, but as seen in our results pages for that model, it worryingly becomes darker with each ISO increment, suggesting a reduction in actual sensitivity.

We should also mention the additional shooting modes of the Sony's which capture and combine multiple frames to reduce noise - which they do very effectively. In addition the Sony's feature automatic panorama generation, in-camera 3D capture (without the need for a separate lens), in-camera HDR, and in the case of the A55, a built-in GPS. Both the Sony SLTs also boast in-camera stabilisation which works with any lens you attach.

The Lumix GH2 is however the more sophisticated movie camera, sporting additional 1080 / 24p and variable speed modes, clean 1080i HDMI output even when filming, adjustable audio levels, a tele-converter which doesn't compromise resolution, and full manual control over exposures.

There's so much to compare and contrast it's almost impossible to recommend one over the other, but having used the GH2 and SLT-A33 extensively side by side, it becomes clear the former feels more sophisticated, especially in terms of movies and displays. But the Sony's take the lead on continuous shooting and their array of innovative shooting modes, not to mention comfortably lower prices. The higher-end A55 boasts GPS and 10fps shooting, yet costs 15% less than the GH2 in a similar kit, while the A33 kit comes across as a bargain at 25-30% cheaper. Independent film makers or most video enthusiasts will however justify the higher cost of the GH2.

It's worth comparing a traditional DSLR against the SLT-A33, and one of the most popular entry-level models with video is the Nikon D3100. Both the D3100 and A33 shoot 14 Megapixel stills and Full HD video using bodies which are roughly similar in size and weight, but there are considerable other differences to weigh-up. Note a fairer comparison in price and specification would arguably be the unannounced successor to the D5000, but we have no information on this product yet.

Both cameras feature 3in screens, although the Sony's is a much higher specification with 1040k dots to 230k on the Nikon, not to mention being fully articulated. As a traditional DSLR, the D3100 employs an optical viewfinder, whereas the SLT-A33 has an electronic viewfinder. Both have their pros and cons. An optical viewfinder is easier to use in low light or when following action, and also lets you 'see' a broader dynamic range of tones; it additionally allows the battery to last much longer, typically capturing three to four times more shots than in Live View alone. In contrast, the A33's Live View Finder delivers a larger image which can enjoy magnified focusing assistance and super-imposed colour graphics like a dual-axis levelling guide. Which is better is entirely personal and dependant on the kind of photos you take.

Staying on the subject of action, the SLT-A33 shoots at roughly twice the speed of the D3100, with 6fps compared to 3 on the Nikon. When set to 6fps (or the aperture-fixed 7fps option), the SLT-A33 doesn't offer a live update between frames, but by dropping it to 2.5fps, it will deliver a live image, while only coming-up a tad slower than the Nikon.

Both cameras employ phase-change autofocus with similar speed, although the Sony enjoys a denser 15-point array to the Nikon's 11-points.

In terms of still photo quality, the D3100 resolves roughly the same amount of detail as the A33, and enjoys an edge at higher ISOs when both cameras are shooting single frames. But the SLT-A33 can lower its visible noise for static subjects with its various Multi Frame options, giving it an advantage over more conventional rivals.

When it comes to video, both cameras offer Full HD recording, albeit with mostly automatic exposure control. The D3100 shoots 1080 progressive at 24fps, while the A33 shoots 1080 interlaced at 50i or 60i depending on your region. Both also offer lower resolution options. In terms of audio, the D3100 only has a mono microphone and no input for external models, whereas the A33 features built-in stereo mics and a 3.5mm jack.

The biggest difference though concerns autofocus while filming. The D3100 claims to offer continuous AF, but does so with a periodic and slow contrast-based search, which frequently hunts back and forth with very distracting results. In contrast the SLT-A33 uniquely employs its phase-change sensor while filming for true continuous AF which is quick and under the right conditions avoids searching.

So far it's looking very much like a one-sided content in favour of the Sony, but the D3100 does come up around 15% cheaper, while offering an arguably better interface and guide for beginners. Traditionalists may also prefer its use of an optical viewfinder.

Ultimately the D3100 remains one of the best entry-level DSLRs which will delight beginners, but the Sony A33 proves spending a little more can get you some significant benefits, especially when it comes to shooting video with autofocus. It'll be interesting to see what Nikon offers during 2011 in the mid-range DSLR category to compete.

Sony's innovative use of a fixed semi-reflective mirror in the SLT-A33 was always going to have pros and cons, but on the whole it delivers a very satisfying experience which in aspects like autofocus during video, is simply leaps and bounds ahead of traditional DSLRs.

Here's a camera with a large sensor and broad catalogue of lenses which really can continuously autofocus during video and deliver an experience close to that of a camcorder. It also presents one of the best Live View experiences to date with a high resolution viewfinder, detailed articulated screen and quick autofocus which lets you get on with picture-taking.

Like Sony's recent models, the SLT-A33 also boasts faster-than-average continuous shooting, which is exploited by a variety of cunning multi-frame modes which combine handheld images to deliver lower noise, higher dynamic range or broad panoramas. We may have seen these before from Sony, but they still impress today and are made all the better on the A33 with its new Multi Frame NR option which lets you manually choose the exposure and ISO.

As discussed in detail above, there are downsides to the system, including a loss of light from the mirror, potential inconsistencies with the phase-change AF sensor, and a reliance on electronic composition which traditionalists, low light or action shooters may not get on with.

Even if you decide a hybrid camera with electronic composition is for you, it's important to remember the Sony SLT is not the only game in town. Panasonic's Lumix GH2 delivers more sophisticated movie capabilities, arguably superior displays and quieter autofocusing from standard lenses. But it also suffers from restrictions regarding continuous AF on many lenses and crucially costs around 50% more.

That's the bottom line with the Sony SLT-A33: it simply delivers terrific value. There may be more sophisticated models available, but few if any match its capabilities at this price point. Sony's thrown down the gauntlet here not just in price but also technology innovation. While most rivals continue to tweak traditional DSLR concepts, Sony's re-invented the hybrid camera, and done so at a highly competitive price-point. If you understand the pros and cons of electronic composition and are happy with the mostly automatic movie mode, the SLT-A33 is very hard to beat and easily comes Highly Recommended.

Good points
Very fast and continuous AF in Live View and HD movies.
High resolution articulated screen and decent EVF.
Fast continuous shooting up to 7fps.
Multi-frame to reduce noise, increase DR or create panoramas.
Continuous AF and stabilisation on entire Alpha lens catalogue.

Bad points
Some light permanently lost due to semi-reflective mirror.
No live image between frames in fast continuous bursts.
Modest buffer size: approx 14 best quality JPEGs.
Most Alpha mount lenses focus quite audibly in movies.
Some will notice rainbow tearing artefacts in viewfinder.

Scores
(relative to 2011
EVIL cameras)

Build quality:
Image quality:
Handling:
Specification:
Value:

Overall:

17 / 2017 / 2016 / 2018 / 2019 / 20

87%

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